My thoughts are that you talk little and listen much. But going in and refusing to say ANYTHING would be sort of silly. Keep very good notes of what was said. In fact, it wouldn't be a bad idea to record the meeting. Bring a tape recorder. N.C. is a one party tape state. Taping can be done without the other party's consent.
State your case. State your belief that the job is not equivalent, and the reasons why. Consider this a formal hearing, not a discussion group. Do not let them "probe you." IF you do not talk too much, make only short clear statements, they can't
back you into a corner. DO NOT argue with them, no matter what sort of ridiculous or threatening or incorrect statement they may make. Do not take it personally if they say you were demoted because your work was not good. Be expecting that one.
Remember, you do not have to answer a question just because it is asked.
Present your case as you have presented it to us here. The part about how before you were on FMLA, you were clearly told you would be taking on more responsibility. How the new position is different. What the different clearances are. State that what you want is your old job back. Do not let them pull you into arguing. Have statements in mind that you wish to make. Do not show excess emotion, resist any impules to tell them off, don't get angry, or in any way indicate that you will or will not pursue the matter further. Ask them what they are planning to do to resolve the situation. Ask when you can expect to hear back from them that they are taking action regarding the situation.
Be quiet, professional, polite. Actually, you do have them rather over a barrel here, if what you are saying is right. They should realize this and be willing to offer you some concessions. It's okay to have it in your mind that you will or will not accept these concessions. But DO NOT decide during the meeting to accept anything they offer without being given the opportunity to take it out and "think about it" before accepting. If they demand you do so, insist you settle with them on the spot, tell them thank you for the offer, but you just cannot do that. If they try to threaten you and say they'll fire you, smile and thank them, tell them you understand their position and leave. Good luck to you. Let us know how it goes.